1. Field of the Invention
The present invention relates to an X-ray imaging apparatus for obtaining an X-ray phase image of a subject, and a wavefront measurement device for measuring a transmission wavefront of X-ray radiation transmitted through the subject using the X-ray imaging apparatus.
2. Description of the Related Art
Phase contrast imaging based on a phase difference of X-ray radiation caused by irradiating a subject with X-rays has been performed since 1990s. Japanese Patent Application Laid-Open No. 2007-203066 corresponding to U.S. Pat. No. 7,486,770 discusses an X-ray phase contrast method (hereinafter referred to as “X-ray Talbot interferometry”) using the so-called Talbot interference. The X-ray Talbot interferometry includes an X-ray radiation source for generating X-ray radiation, a diffraction grating for diffracting the X-ray and forming an interference pattern (hereinafter referred to as a self-image) from a Talbot effect, and an X-ray detector for acquiring an X-ray intensity distribution. When a subject is arranged between the X-ray source and the diffraction grating or between the diffraction grating and the detector, the self-image is modulated by the subject. When the self-image modulated by the subject is detected by the detector, information of the subject can be obtained.
Further, generally, since the self-image has a very small period, it may be difficult to directly detect the self-image. An arrangement of a shielding grating in a position in which the self-image is formed has been discussed. The shielding grating shields a part of the X-ray forming self-image to form a moire. When the moire is detected by a detector, information of the subject can be obtained. A phase grating for modulating a phase may be generally used as the diffraction grating. In the phase grating, phase reference portions and phase shift portions are periodically arranged. The phase shift portion has such a thickness that the phase of the X-ray is shifted by a desired amount when the X-ray is incident on the phase shift portion.
Meanwhile, in the shielding grating, transmission portions that transmit an X-ray and shielding portions that shield the X-ray are periodically arranged. The shielding portion has a sufficient thickness to shield an incident X-ray.
Thus, in the diffraction grating and the shielding grating, the phase shift portions or the shielding portions each having the thickness necessary for their functions are arranged at a fine pitch. Accordingly, the phase shift portion and the shielding portion have a large aspect ratio (i.e., height/width, where the width is the length of the phase shift portion or the shielding portion in the array direction, and the height is the length of the phase shift portion or the shielding portion in the thickness direction of the grids.) Further, in order to increase an imaging range, it is necessary to use a large diffraction grating and a large shielding grating. In the diffraction grating or shielding grating, the X-ray is obliquely incident on the phase shift portion or shielding portion which creates a larger aspect ratio in an area away from the optical axis. Accordingly, the intended functions of the diffraction grating or shielding grating may not be performed and contrast of the formed self-image or moire may be increasingly degraded according to a size of the diffraction grating or shielding grating and the aspect ratio of the phase shift portion or the shielding portion. As a result, measurement accuracy of X-ray phase contrast is degraded as the area is away from the optical axis.
Japanese Patent Application Laid-Open No. 2007-203066 corresponding to U.S. Pat. No. 7,486,770 discusses a method for coping with it, in which a phase shift portion of a diffraction grating and a shielding portion of a shielding grating is processed to be directed to an X-ray source to be in parallel to an incident X-ray, instead of being orthogonal to a grating surface. In the diffraction grating and the shielding grating discussed in Japanese Patent Application Laid-Open No. 2007-203066 corresponding to U.S. Pat. No. 7,486,770, since the phase shift portion and the shielding portion need be directed to a specific direction that depends on a position within a grating surface, fabrication is not easy.